(a) Field of the Invention
The present invention relates to a method of manufacturing hot rolled steel sheets using a mini mill process, and more particularly, to a method in which ultra-thin strip production of hot rolled steel sheets is possible using a mini mill process.
(b) Description of the Related Art
In the mini mill steel making process, the final product is produced in a minimal amount of time using directly connected, short processes starting from a continuous casting process to a rolling process. Accordingly, the mini mill steel making process differs significantly from the blast furnace steel making process.
Although there are many types of mini mill processes, they can be generally divided into two categories depending on the thickness of the resulting slab: a thin slab process in which slabs of less than 70 mm in thickness are produced, and a medium slab process in which slabs of greater than 70 mm in thickness are produced. Also the mini mill process can be divided into two categories depending on the heating and rolling methods used.
As far as the actual method of production is concerned, the typical mini mill processes include the ISP (in-line strip production) process, the CSP (compact strip production) process and the Danieli process.
FIG. 4 shows a schematic view of an ISP process production line. With reference to the drawing, molten steel contained in a ladle 91 is poured into a tundish 92, then passes through a continuous caster 101, having a 75 mm mold, and a liquid core reducer 102. Passing through the continuous caster 101, the molten steel is cast into slabs having a thickness of roughly 60 mm. The slabs, without first being cut to a predetermined length, are descaled in a first descaler 108a, then directly rolled in a reduction unit 103 to produce flat bars having a thickness of 20–30 mm.
After passing through the reduction unit 103, the flat bars are cut to suitable lengths by a first cutter 104a. The cut flat bars are then heated in a heating furnace 105 and coiled in a coiling station 106a. Subsequently, the coiled flat bars are uncoiled in an uncoiler 106b then descaled in a second descaler 108b. Following this process, the flat bars are rolled in a finishing mill 107 to a predetermined final thickness, after which the flat bars are cooled in a cooler 120 and finally coiled in a down coiler 121. Reference numeral 104b in FIG. 4 refers to a second cutter.
In the ISP process described above, since the first cutter 104a is connected downstream from the reduction unit 103, the continuous caster 101 and the reduction unit 103 are in effect connected through the slabs being passed therethrough. Thus it is difficult to control the overall process. Further, since the high temperature slabs cast in the continuous caster 101 are rolled in the reduction unit 103, there will be a possibility of the reduction unit 103 to be deformed by the temperature of the slabs. In addition, the cast slabs are directly rolled in the reduction unit 103 without any heating. As a result, a difference in temperature between edges and a center of the slabs may occur, causing surface defects in the slabs.
In addition, since descaling is performed in the first descaler 108a immediately following continuous casting, optimal-descaling is not achieved. That is, because a scale thickness is limited and there is only a small number of pores on a scale layer, a bonding force between the scales and matrix of the slabs is very high.
With regard to the CSP process, with reference to FIG. 5, molten steel contained in a ladle 91 is poured into a tundish 92 as in the ISP process described above. The molten steel is cast into slabs after passing through a continuous caster 201 and a liquid core reducer 202. The slabs are then cut to suitable lengths by a cutter 204. The cut slabs are heated in a heating furnace 205 having a length of at least 170 m. In the heating furnace 205, the slabs are heated to a temperature suitable for rolling. Following this step, the heated slabs are descaled by a descaler 208 then rolled by six rollers, after which the rolled slabs are cooled by a cooler 220 then coiled by a coiler 221.
In the CSP process described above, because of the considerable length of the heating furnace 205, up to three slabs can be positioned therein at one time. This increases manufacturing productivity. Additionally, since the slabs produced in another continuous caster (not shown) are not directly transmitted to the rollers 207, the heating furnace 205 has to be rotated or moved to feed the slabs into the rollers 207. Another feature of the CSP process is that a reduction unit is not required as in the CSP process since slabs of less than 50 mm in thickness are produced by the continuous caster 201.
However, a drawback of the CSP process is that productivity lags behind other methods which manufacture slabs of medium thickness since casting is done at a faster rate than needed to produce slabs of medium thickness.
Referring now to FIG. 6, illustrating a schematic view of the Danieli process production line, after molten steel in a ladle 91 is poured into a tundish 92, the molten steel being solidified undergoes soft reduction in a 90 mm mold of a continuous caster 301 and a liquid core reducer 302 such that slabs of 70 mm in thickness are produced. The slabs are then cut to suitable lengths by a first cutter 304a. The cut slabs are descaled in a first descaler 308a then heated to a temperature suitable for rolling in a first heating furnace 305. The first heating furnace 305 has a substantial length so that a plurality of slabs can be heated therein at one time.
Because medium slabs are manufactured in the Danieli process, both a roughing mill 303 and a finishing mill 307 are provided. That is, after the slabs are rolled into flat bars by the roughing mill 303, the flat bars undergo rolling also in the finishing mill 307. A heated cover 305b is provided between the roughing mill and finishing mills 303 and 307 to ensure that the flat bars are maintained at an appropriate temperature before being supplied to the finishing mill 307. The length of the heated cover 305b is determined depending on a length of one flat bar. After rolled in the finishing mill 307, the flat bars are cooled by a cooler 320 then coiled by a final coiler 321. Reference numeral 322 in FIG. 6 refers to a width roller, and reference numerals 304b, 308b and 308c refer respectively to a second cutter, a second descaler and a third descaler.
In the Danieli process as described above, because of the extensive length of the heated cover 305b (equal to the length of one slab), an overall length of the Danieli production line is increased.